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The manipulators discussed on this page require the header file named
iomanip.h
#include <iostream.h>
#include <iomanip.h>
Formatting output is important in developing output screens
which can be easily read and understood by the program users. C++ offers
the programmer several
input/output manipulators. Two
of these I/O manipulators are setw and
setprecision.
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The setw
manipulator
sets the width of the field
assigned for the output. It takes the size of the field (in number of characters) as a parameter. |
For example, the code:
cout << setw(6) <<"R";
generates the following output on the screen (each underscore represents a
blank space)
_ _ _ _ _R
The setw( ) is the only manipulator that
does not stick from one command to the next.
If you want to right-justify three numbers with an 8-space output width, you
will need to repeat setw( )
in this manner:
cout << setw(8) << 22 << "\n";
cout << setw(8) << 4444 << "\n";
cout << setw(8) << 666666 << endl;
The output will be (each
underscore represents a blank space)
_ _ _ _ _ _ 2 2
_ _ _ _ 4 4 4 4
_ _ 6 6 6 6 6 6
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The setprecision manipulator
sets the total number of digits to be displayed when floating point numbers are output. |
For example, the code:
cout << setprecision(5) << 123.456;
generates the following output on the screen (notice the
rounding):
123.46
The setprecision manipulator can also
set the number of decimal places
displayed. If you wish to use setprecision in
this manner, you must set an ios flag. The flag is set with the following statement:
cout.setf(ios::fixed);
Once the flag has been set, the number you pass as a parameter to setprecision is the number of decimal places
you wish displayed. For example, the code:
cout.setf(ios::fixed);
cout << setprecision(5) << 12.345678;
generates the following output on the screen (notice no
rounding):
12.34567
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Additional IOS flags:
cout.setf(ios::fixed) // "fixed" is a format option
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Other format options include:
| left |
left-justifies the output |
| right |
right-justifies the output |
| showpoint |
displays decimal point and trailing zeros for all floating point
numbers, even if the decimal places are not needed. |
| uppercase |
displays the "e" in E-notation as "E" rather than "e" |
| showpos |
displays a leading plus sign before positive values |
| scientific |
displays floating point numbers in scientific ("E") notation |
| fixed |
displays floating point numbers in normal notation - no trailing
zeroes and no scientific notation |
**Note: If neither the scientific nor fixed option is set, the compiler decides
upon the method used to display floating-point numbers based on whether the number can be displayed more efficiently in scientific or fixed notation.
**Note: You can remove these options by replacing
setf with unsetf.
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Example:
#include <iostream.h>
int main(void)
{
float x = 18.0;
cout<< x << endl;
//displays 18
cout.setf(ios::showpoint);
cout<< x << endl;
//displays 18.0000
cout.setf(ios::scientific);
cout<< x << endl;
//displays 1.800000e+001
cout.unsetf(ios::showpoint);
cout.unsetf(ios::scientific);
cout<<x<<endl;
//displays 18
return 0;
}
To print 5.8 as 5.80 the following lines of code are
needed
(particularly nice for displaying amounts of money):
cout.setf(ios::fixed);
//Sets the cout flag for fixed-point
//non-scientific notation trailing zeros
cout.setf(ios::showpoint); //Always shows decimal point
cout << setprecision(2); //Two decimal places
cout << 5.8;
| All subsequent couts retain the precision set with the last
setprecision( ). Think of setprecision as being
"sticky." Whatever precision you set sticks with the
cout device until you change it with a subsequent
setprecision( ) later in the program. |
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